I wasn't able to find any studies of using the hot and steamy shower trick at home, but there is plenty of research into the use of humidity to treat croup in the emergency department. In every study, humidity didn't seem to help at all.
Croup causes a loud, barking cough that sounds a bit like a sea lion. It's common in kids and often gets better without treatment.
Healthcare providers have been taught for years that humidity is the cure for croup. As a teacher, I was guilty of perpetuating this myth myself. Indeed, humidity was supposed to work so well; it was actually said that one could diagnose croup if humidity worked to quell the coughing.
Croup that doesn't get better within a couple of days could get dangerously worse. Be sure to take kids to the doctor if their barking cough doesn't go away in a day or two. Call 911 for any person with severe shortness of breath, no matter the cause.
Sources:
Bjornson, C.L., and D.W. Johnson. "Croup in the paediatric emergency department." Paediatr Child Health. Jul 2007
Moore, M., and P. Little. "Humidified air inhalation for treating croup." Cochrane Database Syst Rev. Jul 2006
Scolnik, D., et al."Controlled delivery of high vs low humidity vs mist therapy for croup in emergency departments: a randomized controlled trial." JAMA. Mar 2006

